(1) Field of the Invention
The present invention relates to a congestion relieving system in a communication system, and more particularly, to a congestion relieving system in a broad band ISDN in which data is transferred by using ATM cells, and which is connected to a customer station through a network terminator and a bus conformable to a DQDB protocol.
(2) Description of the Related Art
The research of the broad-band ISDN (Integrated Services Digital Network) has been developed to put or construct an ISDN in which broad-band communication such as video communication and so forth is possible for practical use. The broad-band ISDN is currently realized by using an ATM (Asychronous Transfer Mode). In the ATM, data is transferred by dividing it into fixed-length short packets called cells. The cells are exchanged and transferred by an ATM exchange or a node.
In the ATM, a circuit or, in other words, a line between a sender and a receiver, is not always provided at any time, but a virtual circuit or virtual line is provided through which a necessary ATM cell is transmitted. Therefore, a plurality of virtual circuits commonly have a single transmission line. In this multiplication, a transmission line method called a statistical multiplication is employed. It is necessary to manage the statistical multiplication in such a way that the total data multiplied on a single transmission line does not exceed the capacity of the transmission line. By this, an effect of the multiplication is generated so that the efficient use of the circuit can be increased. Techniques has been developed to treat congestion when it occurs. For subscribers connected to such a broad-band ISDN, a plurality of data receiving/transmitting devices called terminal equipments or terminal adapters in a customer station are connected through a bus. As a configuration of the bus, a DQDB (Distributed Queue Dual Bus) is considered to be favorable. The present invention relates to a congestion relieving system in an ISDN in which the communication between a local switch and each of the terminal equipments or terminal adapters is carried out in accordance with the DQDB protocol.
In the broad-band ISDN communication employing the DQDB protocol in the customer station side, congestion may be generated in an apparatus in the network side. The apparatus in which congestion may be generated may be an ATM exchange, a local switch, or any processing device for processing ATM cells. To relieve congestion generated in the apparatus at the network side, it can be considered to restrict a subscriber, which is sending ATM cells to the apparatus, in which the congestion has occurred, to transmit a cell.
In a conventional congestion relieving system, a network terminator inserts busy bits equal to "1" into busy bit areas of cells in accordance with a required rate of restriction of cells to be transmitted. This conventional system is disclosed in the prior U.S. application Ser. No. 683,043, filed on Apr. 10, 1991. The cells having the busy bits equal to "1" are deemed as those being used by the terminal equipments or the terminal adapters. Therefore, the terminal equipment or terminal adapter does not insert data into these cells with the busy bits ="1". Accordingly, the cells with the busy bits ="1" inserted by the network terminator have no destination so that, when these cells are received by an apparatus such as an ATM exchange and so forth, these cells are deemed invalid and they are discarded. As a result, the number of cells to be transmitted to the apparatus, such as the ATM exchange in which the congestion has been generated, is decreased and the congestion is relieved.
The network terminator is included in the customer station. Therefore, it is desired that the network terminator be as inexpensive as possible. In the above described conventional congestion relieving system, however, there is the disadvantage that the construction of the network terminator located at the up-stream of the up-signal bus is complex. Namely, the network terminator must function to send an empty cell which is without data but with the busy bit ="1" to the up-stream end of the up-signal bus. In other words, the network terminator must function to insert the busy bit ="1" in the cell. This function raises the cost of the network terminator.